1. Field of the Invention
This invention relates to a photosensitive member having excellent photosensitivity characteristics in the visible light region as well as in the near infrared region.
2. Description of the Prior Art
In recent years, the application of amorphous silicon (hereinafter referred to briefly as a-Si), amorphous germanium (hereinafter, a-Ge) and amorphous silicon-germanium (hereinafter, a-Si:Ge) to electrophotographic photosensitive members glow discharge decomposition or sputtering techniques has been gathering attention. This is because photosensitive members containing a-Si, a-Ge and a-Si:Ge are by far superior to the conventional selenium or CdS photosensitive members in terms of freedom from environmental pollution, heat resistance and wear resistance, among others.
Especially in the case of a-Si:Ge, the band gap of Ge is smaller than that of a-Si, so that the addition of an adequate amount of Ge to a-Si can be expected to have the effect of extending the photosensitive range to a longer wavelength, and such extension, if attained, would enable the application of a-Si:Ge to semiconductor laser beam printers now under rapid development. In this connection, when an a-Si:Ge photoconductive layer is used in the form of a single layer structure as above, an increase in the Ge content relative to a-Si will extend the photosensitivity range to a longer wavelength but unfavorably decrease the overall (inclusive of the visible light region) photosensitivity. In other words, Ge is effective in increasing the sensitivity on the longer wavelength side but at the same time it impairs, in a contradictory manner, the excellent visible light region photosensitivity originally owned by a-Si. Therefore, the content of Ge is fairly restricted, and accordingly photosensitive members having desirable photosensitivity characteristics cannot be obtained. Furthermore, Ge is not only high in light absorptivity as compared with a-Si but also low in mobility of charge carriers generated by light absorption. This means that, in the case of a single layer structure, many of the charge carriers are trapped within the photoconductive layer, whereby the residual potential is increased and the photosensitivity decreased in a disadvantageous manner.